Topical anesthetics are products which are applied to skin or mucous tissue of warm-blooded animals to cause temporary numbness at the site of application. One type of topical anesthetic is a liquid evaporative coolant or cold spray which relies on the cooling effect of evaporation to anesthetize nerves. This type of product is used for minor surgical procedures, such as lancing boils or drainage of small abscesses. The application of cold, referred to as cryotherapy, is also used in the treatment of injury or disease, and is particularly widespread in sports medicine.
Ethyl chloride in a spray bottle reportedly has been marketed as a cold spray topical anesthetic since early in the twentieth century, and this product has remained essentially unchanged. When liquid ethyl chloride is sprayed on skin, the ethyl chloride rapidly evaporates and chills the skin to anesthetize the nerves. The cooling effect is due to the heat of vaporization of the boiling liquid. One reason that ethyl chloride is suitable for this purpose is its boiling point of 12.3.degree. C. This boiling point is low enough to cause rapid vaporization on contact with body temperature skin. Yet, the boiling point is high enough to allow the liquid to be kept in a simple sealed container at room temperature, and to allow the anesthetic to be used without damaging the skin.
Currently marketed cold spray anesthetics include pure ethyl chloride spray, as well as blends of ethyl chloride with CFC-114 (dichlorotetrafluoroethane), CFC-114 by itself, and a CFC-11 (trichlorofluoromethane)/CFC-12 (dichlorodifluoromethane) blend. However, there are problems in the use of all of these materials. Because of environmental considerations, chlorofluorocarbons (CFCs) such as CFC-114, CFC-11 and CFC-12 are all being phased out. This leaves only the ethyl chloride coolant spray. However, ethyl chloride is a highly flammable liquid which volatizes at room temperature to form an explosive gas. Upon combustion, ethyl chloride can form hydrogen chloride or even highly toxic phosgene (CCl.sub.2 O). Furthermore, ethyl chloride can react with hot water or steam to form hydrogen chloride or hydrochloric acid. For these reasons, extreme caution must be used when handling and using ethyl chloride.
Thus, there is a need for other materials which can be used as evaporative coolant sprays in place of ethyl chloride, and which are free of the environmentally unacceptable chlorofluorocarbons used in existing products.